PHPLIB-429: Remove support for PHP 5.5

.travis.yml

@@ -22,10 +22,6 @@ env:
 matrix:
   fast_finish: true
   include:
-    - php: 5.5
-      dist: trusty
-      env:
-        - SERVER_DISTRO=ubuntu1404
     - php: 5.6
     - php: 7.0
     - php: 7.1

composer.json

@@ -10,13 +10,13 @@
         { "name": "Katherine Walker", "email": "katherine.walker@mongodb.com" }
     ],
     "require": {
-        "php": ">=5.5",
+        "php": "^5.6 || ^7.0",
         "ext-hash": "*",
         "ext-json": "*",
         "ext-mongodb": "^1.6"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.36 || ^6.4"
+        "phpunit/phpunit": "^5.7.27 || ^6.4"
     },
     "autoload": {
         "psr-4": { "MongoDB\\": "src/" },